Imprinting RF Backscatter on Flexible Surfaces for Next-Generation Sensing

 In this dissertation, we design radio-frequency (RF) backscatter platforms on flexible surfaces to enable next-generation sensing applications. In a typical RF backscatter system, backscatter devices (e.g., sensors and tags) do not actively transmit RF signals as common RF devices would do. Instead, they deliver their message by reflecting and modulating the electromagnetic (EM) waves from a reader device responsible for analyzing the reflected signal and extracting the encoded information. In other words, backscatter devices communicate with their reflections. 

RF backscatter has seen successful implementation and even commercialization in a variety of wireless technologies, for example, Wi-Fi, Bluetooth, mmWave (millimeter-wave), RFID (radio-frequency identification), and Near-field Communication (NFC), enabling a variety of interesting applications. It is attractive because backscatter devices tend to be low-power (or even battery-free), lightweight, and cost-effective. Hence, they are expected to unlock a ubiquitous sensing future where numerous backscatter devices seamlessly blend into our everyday lives, providing continuous and pervasive sensing capabilities, real-time data collection, and enhanced interconnectivity for smart applications. 

Realizing such a future requires integrating wireless sensing capabilities into everyday objects and surfaces, where physical flexibility is key to seamless integration. Since modern RF devices generally break down into rigid printed circuit boards (PCBs), in this dissertation, we seek to fabricate backscatter devices on flexible surfaces instead. We show such novel devices can unlock even more innovative applications and design possibilities in smart buildings, intelligent healthcare, and vehicular/robotic navigation. 

We explore three flexible surfaces in three unique application scenarios. First, we present Textile-Sense, a textile antenna interface for NFC sensing devices. Its soft nature provides maximum flexibility in various human-computer interaction applications such as smart furniture and clothing. Next, we show NFCapsule, a battery-free, ingestible capsule based on near-field coupling for in-body health sensing – detecting the health condition of the human esophagus. Finally, we introduce PolarVisor, where we prototype mmWave metasurfaces with paper and foils and design clutter-free, electronic-free fiducial markers for robotic self-localization applications. We design, fabricate, and deploy these backscatter systems to demonstrate promising results cutting across applications.